The use of etalons for analyzers and/or detectors of coherent radiation is known in the art. As an example, U.S. Pat. No. 4,536,089, entitled "Analyzer for Coherent Radiation", (Aug. 20, 1985) to E. T. Siebert, shows in FIG. 4, a multi-stepped etalon for use with a plurality of radiation detectors coupled to a plurality of detector channels. Reference is also made, by example, to U.S. Pat. No. 4,170,416, entitled "Apparatus for Analyzing Coherent Radiation", (Oct. 9, 1979) to C. R. Fencil. This patent shows a Fabry-Perot interferometer or etalon that comprises a flat glass spacer having partially reflecting, stepped surfaces. The disclosure of each of these two U.S. Patents is incorporated by reference herein in their entireties.
U.S. Pat. No. 3,824,018, (Jul. 16, 1974) to Robert Crane Jr., entitled "Coherent Light Source Detector" is also of interest.
As employed herein, the term "etalon" is intended to encompass an optical device or element having two partially reflecting surfaces that are parallel to each other to optical tolerances. The space between the two reflecting coatings can be air or an optical material, and can be thick or thin. The thicker the spacer, the higher the resolution of the etalon. FIG. 1a shows a "solid" etalon where the spacer is a thick optical material. When the spacer is solid and thin, the etalon assumes the form of an interference filter.
Referring to FIG. 1a, known etalon laser detectors employ an etalon 1 having physical steps (a-d) to generate a phase shift for radiation passing through the etalon. The phase shift across a particular step is given by: EQU .beta.=2ks; EQU k=2.pi.n cos .theta.'/.lambda. (1)
where s=step height; n=index; .theta.'=internal angle; and .lambda.=wavelength.
At midband, .beta. is nominally .pi./2 or 90.degree., which is an optimum value. However, as the wavelength (or angle) changes, the phase moves off of optimum and the modulation (or signal across a step), going as sin .beta., decreases. The same is true of the quadrature phase shift. If this falloff becomes significantly large, proper identification of coherent sources ceases and detection "holes" appear at the bandedges or the field-of-view (FOV) edges.
Known conventional coated laser detection etalons exhibit this undesirable degradation at the bandedges. The degradation is a result of (a) phase changes across the etalon steps, (b) phase changes between in-phase and quadrature channels, which is also due to phase changes across an etalon step, and (c) phase changes due to the FOV.
For conventional coatings the typical falloff of modulation with wavelength/FOV is shown in FIG. 1b. This falloff is a significant contributor to the generation of (a) detection holes in the field of view, (b) limited spectral coverage, (c) and a requirement for additional detector channels, which adds cost and complexity to the laser detection system.
It is thus one object of the invention to provide improved coatings for laser detection etalons that eliminate or minimize the undesirable degradation at the band edges due to the aforementioned phase changes at the etalon steps.